Automatic transmission control system

ABSTRACT

An automatic transmission comprises a plurality of clutches  11˜15,  which are provided to select power transmission paths, respectively, in a parallel shaft speed change mechanism TM and a plurality of shift valves  60˜68,  which are used for controlling the supply and drain of hydraulic pressure to and from these clutches. For this automatic transmission, a control system comprises a forward/reverse selection hydraulic servomechanism 70 and a plurality of solenoid valves  81˜85,  which are used for controlling the supply and drain of a line pressure so as to control the actuation of these shift valves and the forward/reverse selection hydraulic servomechanism. The system further comprises a hydraulic switch  93  which detects the hydraulic pressure supplied into the right side oil chamber  73  of the forward/reverse selection hydraulic servomechanism  70  and a hydraulic switch  92  which detects the hydraulic pressure of the SECOND speed clutch  12,  whose pressure is generated from the pressure in the right side oil chamber  73.  The system determines whether the forward drive range is set correctly or not, based on a result of pressure detection by these hydraulic switches  92  and  93.

FIELD OF THE INVENTION

The present invention relates to an automatic transmission whichcomprises a power transmission mechanism incorporating a plurality ofpower transmission paths and a plurality of hydraulically operatedfrictionally engaging means. These hydraulically operated frictionallyengaging means are controlled to select these power transmission pathsindividually, by means of hydraulic oil which is supplied through aplurality of shift control valves.

BACKGROUND OF THE INVENTION

Such automatic transmissions have been known and are utilized, forexample, as automatic transmissions for vehicles. Typically, anautomatic transmission designed for use in a vehicle operates such thatthe transmission automatically controls the actuation of hydraulicclutches to change the speed change ratio in correspondence to thedriving condition of the vehicle. Generally, the automatic transmissionincludes a speed change hydraulic unit which comprises a plurality ofshift control valves, a solenoid valve to control the actuation of theseshift control valves, and a manual valve operated in correspondence tothe movement of the shift lever manipulated by a driver. In thisarrangement, the automatic transmission automatically performs speedchange control for a plurality of ranges, i.e., a reverse drive range, aneutral range and a forward drive range (D range, Second, First, etc.)which are individually selected by the operation of the manual valveoperated in correspondence to the manipulation of the shift lever(generally, this automatic control is effective only in the forwarddrive range).

Recently, another type of automatic transmission has been introduced.This automatic transmission does not use a manual valve for theselection of a range and controls the shift of speed change ratios ineach range solely by electrical signals. Such transmissions aredisclosed in Japanese Laid-Open Patent Publication Nos. H5 (1993)-209683(A) and H5 (1993)-215228 (A). Each automatic transmission disclosedthere comprises a plurality of solenoid valves to control the actuationof shift control valves. The selection of the ranges, i.e., the forwarddrive range, the neutral range, and the reverse drive range, as well asthe selection of the speed change ratios in the forward drive range areperformed in correspondence to the command signals (electrical signals)which actuate these solenoid valves.

In such a speed change control system, as the selecting or switching ofdrive ranges and the setting of speed change ratios are controlledsolely by the control pressures which are supplied from solenoid valves,if a solenoid valve malfunctions, then there can be a problem ofinaccuracy in the selection of the drive ranges. As a preparation forsuch a problem, Japanese Laid-Open Patent Publication No. H5(1993)-223156 (A) discloses a method for determining the drive ranges.In this method, hydraulic sensors are provided for detecting the controlpressures that are generated for the respective drive ranges, andthrough the detection of the control pressures, the drive range selectedactually at the moment is determined. In addition, drive rangedetermination means determines the momentary drive range from thesignals used for actuating the solenoid valves, and the drive rangedetermined as presently selected from the actuation signals of thesolenoid valves is compared with the drive range determined as actuallyselected from the detection of the control pressures, to confirm theselection of a right drive range and to determine whether there is anoccurrence of abnormality.

Because a plurality of hydraulic sensors are provided to detect thecommand pressures that are generated for establishing the respectivedrive ranges, this method is likely to be affected from another problem.If any one of the hydraulic sensors malfunctions, then the determinationof the drive ranges and of abnormality will become unreliable. To solvethis problem, spare hydraulic sensors can be provided for each driverange. However, such solution is very costly because the sensorsprovided in a large number are used only for the detection ofabnormality in the selection of the drive ranges.

SUMMARY OF THE INVENTION

To solve the above mentioned problem, the present invention provides acontrol system for an automatic transmission, which system performs areliable determination of abnormality by determining whether a driverange is selected correctly. This system according to the presentinvention is capable of utilizing hydraulic pressure detecting meansused for the above mentioned abnormality detection, for other purposes.

In order to achieve these objectives, the present invention provides acontrol system for an automatic transmission. This system comprises apower transmission mechanism (for example, the parallel shaft speedchange mechanism TM of the embodiment described in the followingsection), a plurality of hydraulically operated frictionally engagingmeans (for example, the LOW clutch 11, the SECOND speed clutch 12, theTHIRD speed clutch 13, the FOURTH speed clutch 14 and the FIFTH speedclutch 15 of the following embodiment), a plurality of shift controlvalves (for example, the first shift valve 60, the second shift valve62, the third shift valve 64, the fourth shift valve 66, the fifth shiftvalve 68, the CPB valve 56 and the D inhibitor valve 58 of the followingembodiment) and a forward/reverse hydraulic servomechanism (for example,the forward/reverse selection hydraulic servomechanism 70 of thefollowing embodiment). Here, the power transmission mechanismincorporates a plurality of power transmission paths, and thehydraulically operated frictionally engaging means are activated by thesupply of hydraulic pressure regulated by the shift control valves forthe selection of the power transmission paths individually in the powertransmission mechanism. The forward/reverse hydraulic servomechanismselects either the power transmission paths which belong to the forwarddrive range or the power transmission paths which belong to the reversedrive range. The control system further comprises a plurality ofsolenoid valves (for example, the first˜fifth on/off solenoid valves81˜85 of the following embodiment) for supplying and draining a linepressure. In this arrangement, the line pressure supplied and drainedthrough the solenoid valves is used to actuate the shift control valvesand the forward/reverse hydraulic servomechanism for selecting the powertransmission paths in speed change control. The system also comprisesfirst pressure detecting means (for example, the hydraulic switch 93 ofthe following embodiment) and second pressure detecting means (forexample, the hydraulic switch 92 of the following embodiment). The firstpressure detecting means detects the hydraulic pressure which issupplied to a forward drive servo-oil chamber (for example, the rightside oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70 of the following embodiment) for selecting the powertransmission paths of the forward drive range. The second pressuredetecting means detects the hydraulic pressure which is supplied to oneof the hydraulically operated frictionally engaging means (for example,the SECOND speed clutch 12 of the following embodiment), utilizing thehydraulic pressure supplied to the forward drive servo-oil chamber, forestablishing a specific speed ratio for the forward drive range. Withthis construction, the control system can determine whether the forwarddrive range is set correctly or not, based on a result of pressuredetection by the first and second pressure detecting means.

In this arrangement, the first pressure detecting means detects thehydraulic pressure supplied to the forward drive servo-oil chamber ofthe forward/reverse hydraulic servomechanism while the second pressuredetecting means detects the hydraulic pressure supplied from the forwarddrive servo-oil chamber to a specific hydraulically operatedfrictionally engaging means. Therefore, whether the line pressure issupplied to the forward drive servo-oil chamber or not is determinablefrom the result of the detection by either of the two pressure detectingmeans. Even if one of the pressure detecting means malfunctions, themonitoring of the actuation of the forward/reverse hydraulicservomechanism can be continued by the other pressure detecting means.Furthermore, because of these two pressure detecting means, the systemis capable of recognizing the fault or breakdown of the pressuredetecting means and the fault or breakdown of the hydraulic circuit asseparate phenomena, so the system can control the transmission in a modewhich is appropriate for the recognized fault or breakdown.

In addition to the detection of the actuation of the forward/reversehydraulic servomechanism described above, the first pressure detectingmeans can be also used to detect the actuation of valves which controlthe supply of pressure to the forward drive servo-oil chamber. Also, thesecond pressure detecting means can be used to detect the actuation ofvalves which control the supply of pressure from the forward driveservo-oil chamber to a specific hydraulically operated frictionallyengaging means. Therefore, the timing for setting a target speed ratiofor the forward drive range can be controlled on the basis of the resultof detection by the second pressure detecting means.

Preferably, a D inhibitor valve and a predetermined shift control valve(for example, the fourth shift valve 66 of the following embodiment),each of which is retainable at a respective forward drive position forthe forward drive range, are arranged in this order on a servo-pressuresupply line (for example, the oil passages 101 b, 101 e, 126 and 125 ofthe following embodiment) which connects a source of the line pressure(for example, the oil pump OP and the main regulator valve 50 of thefollowing embodiment) to the forward drive servo-oil chamber. In thisarrangement, the servo-pressure supply line is in fluid communicationwhen the D inhibitor valve and the predetermined shift control valve arepositioned at the respective forward drive positions, and the firstpressure detecting means detects the hydraulic pressure of theservo-pressure supply line at a location between the D inhibitor valveand the predetermined shift control valve. With this arrangement, thesystem can monitor the actuation of the D inhibitor valve by the firstpressure detecting means and can control the timing to switch from theforward drive range to a neutral range, based on the result of thedetection by the first pressure detecting means.

Preferably, when the forward drive range is judged not being setcorrectly, based on the result of pressure detection by the first andsecond pressure detecting means, the control system controls thesolenoid valves in accordance to modes which are predetermined forpossible faults and breakdowns and thereby sets the transmission into arelief drive mode.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawings,which are given by way of illustration only and thus are not limitativeof the present invention.

FIG. 1 is a sectional view of an automatic transmission whose speedchange is controlled by a control system according to the presentinvention.

FIG. 2 is a partial sectional view of the automatic transmission.

FIG. 3 is a skeleton diagram which shows schematically the powertransmission system of the automatic transmission.

FIG. 4 is a schematic diagram showing the relative positions of theshafts of the automatic transmission.

FIG. 5 is a diagram showing a hydraulic circuit which constitutes acontrol system according to the present invention.

FIGS. 6˜10 are diagrams, each showing part of the diagram of FIG. 5,respectively, in enlargement.

FIG. 11 is a flowchart showing steps of an abnormality determinationcontrol which is executed by the control system.

FIG. 12 is d flowchart showing steps of a speed change control executedby the control system.

FIG. 13 is a list showing results of the abnormality determination,which are stored in memory.

FIG. 14 is a flowchart showing steps of a D range relief drive control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 4 show an automatic transmission which incorporates acontrol system according to the present invention. In a transmissionhousing HSG, this transmission comprises a torque converter TC, which isconnected to the output shaft of an engine (not shown), a parallel shaftspeed change mechanism TM, which is connected to the output member (orturbine) of the torque converter TC, and a differential mechanism DFincluding a last reduction driven gear 6 b, which meshes with a lastreduction drive gear 6 a provided in the speed change mechanism TM. Thedrive power for the vehicle is transmitted through the differentialmechanism DF to lateral wheels.

The parallel shaft speed change mechanism TM includes a first inputshaft 1, a second input shaft 2, a countershaft 3, and an idle shaft 5,all of which are disposed parallel with one another. FIG. 4 shows thepositions of these shafts in the housing, the centers of the shaftsbeing indicated by corresponding alphanumeric marks, S1, S2, S3 and S5.FIGS. 3A and 3B show the rotational components of the speed changemechanism TM, which are arranged for mechanical power transmission. FIG.3A is a schematic sectional view showing the first input shaft 1 (S1),the second input shaft 2 (S2) and the countershaft 3 (S3), which aretaken along line IIIA—IIIA in FIG. 4, while FIG. 3B is a schematicsectional view showing the first input shaft 1 (S1 ), the second inputshaft 2 (S2) and the idle shaft 5 (S5), which are taken along lineIIIB—IIIB in FIG. 4. Furthermore, FIG. 1 corresponds with FIG. 3A whileFIG. 2 corresponds with FIG. 3B, all of which are sectional views of thespeed change mechanism TM.

The first input shaft 1 is connected directly to the turbine of thetorque converter TC and is supported rotatably by bearings 41 a and 41b. The first input shaft 1 receives the drive power from the turbine androtates with it at the same rotational speed. On this input shaft 1,from the side of the torque converter TC (i.e., the right side of thedrawing), disposed are a FIFTH speed drive gear 25 a, a FIFTH speedclutch 15, a FOURTH speed clutch 14, a FOURTH speed drive gear 24 a, areverse drive gear 26 a, and a first connection gear 31. The FIFTH speeddrive gear 25 a is disposed rotatably on the first input shaft 1, andthe FIFTH speed clutch 15, which is actuated hydraulically, engages withthe FIFTH speed drive gear 25 a to connect it rotationally to the firstinput shaft 1. The FOURTH speed drive gear 24 a and the reverse drivegear 26 a, which are coupled as one body, are disposed rotatably on thefirst input shaft 1, and the FOURTH speed clutch 14, which is actuatedhydraulically, engages with these gears to connect them rotationally tothe first input shaft 1. The first connection gear 31 is mounted on thefirst input shaft 1, at the left end thereof outside the bearing 41 a,which supports the first input shaft 1 rotatably. In this condition, thefirst connection gear 31 and this end portion of the first input shaft 1are supported only by this bearing 41 a in cantilever.

The second input shaft 2 is also supported rotatably by bearings 42 aand 42 b. On this input shaft 2, from the right side of the drawing,disposed are a SECOND speed clutch 12, a SECOND speed drive gear 22 a, aLOW drive gear 21 a, a LOW clutch 11, a THIRD speed clutch 13, a THIRDspeed drive gear 23 a, and a fourth connection gear 34. The SECOND speeddrive gear 22 a, the LOW drive gear 21 a and the THIRD speed drive gear23 a are each disposed rotatably on the second input shaft 2, and theSECOND speed clutch 12, the LOW clutch 11, or the THIRD speed clutch 13,which is actuated hydraulically, engages with the respective gear toconnect it rotationally to the second input shaft 2. In addition, thefourth connection gear 34 is coupled to the second input shaft 2.

The idle shaft 5 including a second connection gear 32 and a thirdconnection gear 33, which are formed as one body with the idle shaft 5,is supported rotatably by bearings 45 a and 45 b. The second connectiongear 32 meshes with the first connection gear 31 while the thirdconnection gear 33 meshes with the fourth connection gear 34. The first,second, third and fourth connection gears constitute a connection geartrain 30, through which the rotation of the first input shaft 1 istransmitted continually to the second input shaft 2.

The countershaft 3 is supported rotatably by bearings 43 a and 43 b. Onthis shaft 3, from the right side of the drawing, disposed are the abovementioned last reduction drive gear 6 a, a SECOND speed driven gear 22b, a LOW driven gear 21 b, a FIFTH speed driven gear 25 b, a THIRD speeddriven gear 23 b, a FOURTH speed driven gear 24 b, a dog clutch 16, anda reverse driven gear 26 c. The last reduction drive gear 6 a, theSECOND speed driven gear 22 b, the LOW driven gear 21 b, the FIFTH speeddriven gear 25 b and the THIRD speed driven gear 23 b are fixed on androtate together with the countershaft 3 while the FOURTH speed drivengear 24 b and the reverse driven gear 26 c are each disposed rotatablyon the countershaft 3. The dog clutch 16 is actuated axially in onedirection to engage with the FOURTH speed driven gear 24 b so as toconnect it rotationally to the countershaft 3 or in the oppositedirection to engage with the reverse driven gear 26 c so as to connectit rotationally to the countershaft 3.

As shown in the drawings, the LOW drive gear 21 a meshes with the LOWdriven gear 21 b; the SECOND speed drive gear 22 a meshes with theSECOND speed driven gear 22 b; the THIRD speed drive gear 23 a mesheswith the THIRD speed driven gear 23 b; the FOURTH speed drive gear 24 ameshes with the FOURTH speed driven gear 24 b; and the FIFTH speed drivegear 25 a meshes with the FIFTH speed driven gear 25 b. In addition, thereverse drive gear 26 a meshes with a reverse idler gear 26 b (refer toFIG. 2), which then meshes with the reverse driven gear 26 c.

The last reduction drive gear 6 a meshes with the last reduction drivengear 6 b (refer to FIG. 1, which shows that they are situated at thesame position in the axial direction though the drawing does not showthe actual condition that they mesh with each other). The rotation ofthe countershaft 3 is transmitted through the last reduction drive anddriven gears 6 a and 6 b to the differential mechanism DF.

Now, a description is given of how each speed ratio is established andthrough which path the drive power is transmitted at each speed ratio.In this transmission, for establishing the forward drive range, the dogclutch 16 is shifted toward the right side of the drawing, where the dogclutch 16 engages with the FOURTH speed driven gear 24 b to connect itrotationally to the countershaft 3. For the reverse drive range, the dogclutch 16 is shifted leftward, where the dog clutch 16 engages with thereverse driven gear 26 c to connect it rotationally to the countershaft3.

First, the establishment of each speed ratio of the forward drive rangeis described. The LOW ratio is established when the LOW clutch 11 isengaged. The rotational drive power which is input from the torqueconverter TC to the first input shaft 1 is transmitted through theconnection gear train 30 to the second input shaft 2. Because the LOWclutch 11 is engaged, the LOW drive gear 21 a which is driven at thesame rotational speed as the second input shaft 2 drives the LOW drivengear 21 b mounted on the countershaft 3. This drive power is thentransmitted through the last reduction drive and driven gears 6 a and 6b to the differential mechanism DF.

The SECOND speed ratio is established when the SECOND speed clutch 12 isengaged. The rotational drive power which is input from the torqueconverter TC to the first input shaft 1 is transmitted through theconnection gear train 30 to the second input shaft 2. Because the SECONDspeed clutch 12 is engaged, the SECOND speed drive gear 22 a which isdriven at the same rotational speed as the second input shaft 2 drivesthe SECOND speed driven gear 22 b mounted on the countershaft 3. Thisdrive power is then transmitted through the last reduction drive anddriven gears 6 a and 6 b to the differential mechanism DF.

The THIRD speed ratio is established when the THIRD speed clutch 13 isengaged. The rotational drive power which is input from the torqueconverter TC to the first input shaft 1 is transmitted through theconnection gear train 30 to the second input shaft 2. Because the THIRDspeed clutch 13 is engaged, the THIRD speed drive gear 23 a which isdriven at the same rotational speed as the second input shaft 2 drivesthe THIRD speed driven gear 23 b mounted on the countershaft 3. Thisdrive power is then transmitted through the last reduction drive anddriven gears 6 a and 6 b to the differential mechanism DF.

The FOURTH speed ratio is established when the FOURTH speed clutch 14 isengaged. The rotational drive power which is input from the torqueconverter TC to the first input shaft 1 is transmitted through theFOURTH speed clutch 14 to the FOURTH speed drive gear 24 a, which drivesthe FOURTH speed driven gear 24 b. Because the dog clutch 16 is keptengaged with the FOURTH speed driven gear 24 b for the forward driverange, the countershaft 3 is driven. This drive power is thentransmitted through the last reduction drive and driven gears 6 a and 6b to the differential mechanism DF.

The FIFTH speed ratio is established when the FIFTH speed clutch 15 isengaged. The rotational drive power which is input from the torqueconverter TC to the first input shaft 1 is transmitted through the FIFTHspeed clutch 15 to the FIFTH speed drive gear 25 a, which drives theFIFTH speed driven gear 25 b. The FIFTH speed driven gear 25 b, which isfixed to the countershaft 3, in turn, drives the countershaft 3. Thisdrive power is then transmitted through the last reduction drive anddriven gears 6 a and 6 b to the differential mechanism DF.

The reverse drive range is established when the FOURTH speed clutch 14is engaged, and the dog clutch 16 is shifted leftward. The rotationaldrive power which is input from the torque converter TC to the firstinput shaft 1 is transmitted through the FOURTH speed clutch 14 to thereverse drive gear 26 a , which, in turn, drives the reverse driven gear26 c through the reverse idler gear 26 c. Because the dog clutch 16 isengaged with the reverse driven gear 26 c for the reverse drive range,the countershaft 3 is driven. This drive power is then transmittedthrough the last reduction drive and driven gears 6 a and 6 b to thedifferential mechanism DF. It should be noted that, as described in thisparagraph, the FOURTH speed clutch 14 is used as a reverse clutch forthe establishment of the reverse drive range in this transmission inaddition to the establishment of the above described FOURTH speed ratio.

Now, in reference to FIGS. 5˜10, a description is made of a hydrauliccircuit which constitutes a speed change control system in thisautomatic transmission. FIGS. 6˜10 show five sections of the hydrauliccircuit at an enlarged scale, which sections are partitioned byalternate long and short dash lines A˜E, respectively, in FIG. 5. Thepoints of the oil passages shown open in the hydraulic circuit diagramare connected to a drainage system.

This hydraulic circuit includes an oil pump OP being driven by theengine to supply working oil from an oil tank OT to an oil passage 100.This oil passage 100 is connected through a branch passage 100 a to amain regulator valve 50, where the pressure of the oil in the oilpassages 100 and 100 a is adjusted to a predetermined line pressure PL.This line pressure PL is then supplied through another branch passage100 b to first˜fifth on/off solenoid valves 81˜85 and to a first linearsolenoid valve 86.

Surplus oil from the oil used for the generation of the line pressure PLat the main regulator valve 50 is led to an oil passage 101 and then toanother oil passage 102. The oil flowing to the passage 101 is regulatedby a lock-up shift valve 51, a lock-up control valve 52 and a torqueconverter check valve 53, and the oil is used for actuating and lockingup the torque converter TC. After being used for the control of thetorque converter TC, this oil returns through an oil cooler 54 to theoil tank OT. In this description, no explanation is given of the controlof the torque converter TC because the control of the torque converteris not related directly to the present invention. The pressure of theoil supplied to the passage 102 is adjusted by a lubrication reliefvalve 55, and this oil is used for lubricating various parts of thetransmission.

As shown in FIG. 5, a LOW accumulator 75, a SECOND accumulator 76, aTHIRD accumulator 77, a FOURTH accumulator 78 and a FIFTH accumulator 79are connected through oil passages, respectively, to the LOW clutch 11,the SECOND speed clutch 12, the THIRD speed clutch, the FOURTH speedclutch 14 and the FIFTH speed clutch 15, which constitute the automatictransmission as described above. This hydraulic circuit is also equippedwith a forward/reverse selection hydraulic servomechanism 70 to operatethe dog clutch 16.

Furthermore, as shown in the figure, a first shift valve 60, a secondshift valve 62, a third shift valve 64, a fourth shift valve 66, a fifthshift valve 68, a CPB valve 56 and a D inhibitor valve 58 are providedto control the hydraulic pressure supplied to these clutches 11˜15 andto the forward/reverse selection hydraulic servomechanism 70. To controlthe actuation of these valves and to control the hydraulic pressuresupplied to the clutches, etc, the above mentioned first˜fifth on/offsolenoid valves 81˜85 and the first˜third linear solenoid valves 86˜88are arranged appropriately.

Now, the operation of this hydraulic circuit is described for each speedchange ratio, which is established when the condition of the first˜fifthon/off solenoid valves 81˜85 is set as listed in Table 1 below. Thefirst˜fifth on/off solenoid valves 81˜85 are normally closed valves, soeach valve opens to generate a signal pressure to actuates otherrespective valves when its solenoid is electrically energized (i.e.,while it is turned ON).

TABLE 1 Solenoid valves 81 82 83 84 85 Mode R X X X ◯ ◯ Reverse N ◯ ◯ ◯◯ ◯ First NEUTRAL ◯ X X ◯ X Second NEUTRAL D ◯ ◯ ◯ ◯ X F/S SECOND X ◯ ◯X X In-gear ◯ ◯ ◯ X X LOW X ◯ ◯ X ◯/X 1-2-3 X ◯ X X ◯/X SECOND X X X X◯/X THIRD X X ◯ X ◯/X 2-3-4 ◯ X ◯ X ◯/X FOURTH ◯ X X X ◯/X 4-5 ◯ ◯ X X◯/X FIFTH Note: “◯” and “X” in the table represent the turning on andoff of the solenoids, respectively.

At first, a description is given for the establishment of the reversespeed ratio. As shown in Table 1, the first˜third on/off solenoid valves81˜83 are turned off and are closed while the fourth and fifth on/offsolenoid valves 84 and 85 are turned on and are opened. In thiscondition, the line pressure PL which is supplied to the fourth andfifth on/off solenoid valves 84 and 85 through oil passages 101 b and101 c that branch from the oil passage 100 b is supplied to oil passages102 and 103. The line pressure PL in the passage 102 acts on the rightend flange portion of the fourth shift valve 66 through an oil passage102 a and shifts the spool 66 a of the valve rightward (this actionresults in a condition opposite to that shown in the figure). The linepressure PL in the passage 103 acts on the left end of the fifth shiftvalve 68 and shifts the spool 68 a of the valve rightward (this resultsin a condition opposite to that shown in the figure). As a result, anoil passage 102 b that is branched from the passage 102 is blocked atthe fifth shift valve 68.

On the other hand, the line pressure PL being supplied to the fifthshift valve 68 through an oil passage 101 e that is branched from thepassage 100 b is supplied through a groove provided on the spool 68 a ofthe fifth shift valve 68 to an oil passage 104, which leads to the Dinhibitor valve 58. In this condition, because an oil passage 105connected to the left end of the D inhibitor valve 58 is connected to adrain at the first on/off solenoid valve 81, the spool 58 a of the Dinhibitor valve 58 is positioned to the left side of the valve, so thepassage 104 is connected with a passage 106 which is connected to theleft side oil chamber 72 of the forward/reverse selection hydraulicservomechanism 70. Therefore, the line pressure PL being supplied intothe left side oil chamber 72 pushes rightward the piston portion 71 a ofa rod 71 which is provided in the forward/reverse selection hydraulicservomechanism 70. When the rod 71, which is provided with a shift forkto operate the dog clutch 16, is shifted rightward, the dog clutch 16engages with the reverse driven gear 26 c to connect it rotationally tothe countershaft 3.

As mentioned previously, the reverse drive range is established when thedog clutch 16 is engaged with the reverse driven gear 26 c and theFOURTH speed clutch 14 is engaged. The engagement of the FOURTH speedclutch 14 is actuated by the first linear solenoid valve 86, to whichthe line pressure PL is supplied through an oil passage 101 d. At thefirst linear solenoid valve 86, the supply of the line pressure toanother oil passage 107 is adjusted by means of electrical control ofthe current flowing through the solenoid of the valve (pressureadjustment control).

This passage 107 is connected with another oil passage 108 through theCPB valve 56, and this oil passage 108 is connected with another oilpassage 109 through a passage which is created when the spool 68 a ofthe fifth shift valve 68 is shifted rightward. This oil passage 109 isthen connected with another oil passage 110 through a passage which iscreated by a groove of the rod 71 of the forward/reverse selectionhydraulic servomechanism 70 when the rod is shifted rightward. This oilpassage 110 is then connected with another oil passage 111 through apassage which is created when the spool 66 a of the fourth shift valve66 is shifted rightward. Then, this oil passage 111 is connected withanother oil passage 112 through a passage which is created when thespool 60 a of the first shift valve 60 is shifted rightward. This oilpassage 112 is then connected with another oil passage 113 through apassage which is created when the spool 64 a of the third shift valve 64is shifted rightward. Furthermore, this oil passage 113 is connectedwith another oil passage 114 through a passage which is created when thespool 62 a of the second shift valve 62 is shifted rightward. This oilpassage 114 is then connected to the actuation oil chamber of the FOURTHspeed clutch 14 and to the FOURTH accumulator 78. In this arrangement,the engagement of the FOURTH speed clutch 14 is controlled by the firstlinear solenoid valve 86 for setting the reverse speed ratio.

Now, the control for establishing the neutral range is described. Asshown in Table 1, the neutral range comprises first and Second NEUTRALmodes. The First NEUTRAL mode takes place when the neutral range (Nrange) or the reverse drive range (R range) is selected while thevehicle is driving at a speed greater than a predetermined speed (e.g.,10 km/h) in the forward drive range (D range). The First NEUTRAL modefunctions as a reverse inhibitor to prevent the transmission fromshifting to the reverse speed ratio in such a condition. The SecondNEUTRAL mode takes place when the transmission shifts from the reversedrive range to the neutral range and from the forward drive range to theneutral range. Moreover, when the transmission shifts from the reversedrive range through the Second NEUTRAL mode to the forward drive range,the transmission goes through the In-gear mode listed in Table 1. On theother hand, if the transmission, after having shifted from the reversedrive range to the Second NEUTRAL mode, is operated to shift from theSecond NEUTRAL mode to the reverse drive range, the transmission shiftsdirectly to the reverse drive range without going through the In-gearmode. In the same way, the Second NEUTRAL mode takes place when thetransmission shifts from the forward drive range to the reverse driverange, so the reverse drive range is established after the D inhibitorvalve 58 is actuated to a REVERSE mode. However, if the transmission,after having shifted from the forward drive range to the Second NEUTRALmode, is operated to shift from the Second NEUTRAL mode to the forwarddrive range, the transmission shifts directly to the forward drive rangewithout any mode change of the D inhibitor valve 58.

In the First NEUTRAL mode, all the first˜fifth on/off solenoid valves81˜85 are turned on and are open. Therefore, when the mode of thetransmission is changing from the reverse speed ratio or reverse driverange to the First NEUTRAL mode, the first˜third on/off solenoid valves81˜83, which have been closed for setting the reverse drive range, arenow opened, and the supply of hydraulic oil through these valves starts.At first, the line pressure PL which is supplied to the first on/offsolenoid valve 81 through the oil passage 101 a is now led to an oilpassage 122 which is connected to the right end of the first shift valve60. With this supply of the line pressure PL, the spool 60 a of thefirst shift valve 60 is shifted leftward. As the oil passage 122 isconnected also with the oil passage 105, the line pressure is suppliedalso to the left end of the D inhibitor valve 58 through the passage105. As a result, the spool 58 a of the D inhibitor valve shiftsrightward. In this condition, the passage 106 which is connected to theleft side oil chamber 72 of the forward/reverse selection hydraulicservomechanism 70 is connected to a drain through the D inhibitor valve58, so the hydraulic oil in the left side oil chamber 72 is drained.

In the condition where the spool 58 a of the D inhibitor valve 58 isshifted to the right side thereof, the line pressure PL is supplied intothe D inhibitor valve 58 through the oil passages 101 e and 135, andthis pressure acts on the spool 58 a to maintain the spool on the rightside of the valve even after the line pressure supplied from the oilpassage 105 is terminated. To the D inhibitor valve 58, another oilpassage 139 is arranged such that the line pressure led through thispassage acts on the spool 58 a to shift the spool leftward. Therefore,only when the line pressure is led through this oil passage 139, thespool 58 a of the D inhibitor valve 58 can shift leftward.

Also, the line pressure supplied through the oil passage 101 a to thesecond on/off solenoid valve 82 is now led to the oil passage 121 whichis connected to the right end of the second shift valve 62. With thissupply of hydraulic pressure, the spool 62 a of the second shift valve62 shifts leftward. Furthermore, the line pressure supplied through theoil passage 101 b to the third on/off solenoid valve 83 is led to theoil passage 123 which is connected to the right end of the third shiftvalve 64. With this pressure, the spool 64 a of the third shift valve 64shifts leftward. As a result, the oil passage 114 connected to the oilchamber of the FOURTH speed clutch 14 is now connected to a drainthrough a groove provided on the spool of the second shift valve 62, andthe FOURTH speed clutch 14 is released to set a neutral condition.

In this condition, as mentioned above, the spool 58 a of the D inhibitorvalve 58 is shifted rightward and positioned at the forward driveposition, so the passage 106 connected to the left side oil chamber 72of the forward/reverse selection hydraulic servomechanism 70 is in fluidcommunication to a drain at the D inhibitor valve 58. Furthermore, theoil passage 125 which is connected to the right side oil chamber 73 isalso in fluid communication to a drain through the fourth shift valve 66whose spool 66 a is shifted rightward. As a result, in the First NEUTRALmode, no force acts on the rod 71 of the forward/reverse selectionhydraulic servomechanism 70 in the axial direction, so the rod 71remains in the same condition which has existed prior to theestablishment of the First NEUTRAL mode.

Here, as an oil passage 126 is connected to a hydraulic switch 93through an extension 126 a branching from the passage 126, when thespool 58 a of the D inhibitor valve 58 is shifted rightward, the linepressure of the oil passage 101 e supplied to the oil passage 126through the D inhibitor valve 58 is detected by the hydraulic switch 93.In other words, whether the supply of the line pressure into the rightside oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70 is ready or not is detectable by the hydraulic switch93.

In the Second NEUTRAL mode, the first and fourth on/off solenoid valves81 and 84 are turned on and are open while the second, third and fifthon/off solenoid valves 82, 83 and 85 are turned off and are closed. Inresponse to this actuation condition of the solenoid valves, the shiftvalves are set as follows: the spool 60 a of the first shift valve 60 isshifted leftward, the spool 62 a of the second shift valve 62 is shiftedrightward, the spool 64 a of the third shift valve 64 is shiftedrightward, the spool 66 a of the fourth shift valve 66 is shiftedrightward, and the spool 68 a of the fifth shift valve 68 is shiftedleftward.

In this condition, as the spool 66 a of the fourth shift valve 66 isshifted rightward, the oil passage 125 connected to the right side oilchamber 73 of the forward/reverse selection hydraulic servomechanism 70is connected to a drain at the fourth shift valve 66. On the other hand,the passage 106 connected to the left side oil chamber 72 of theforward/reverse selection hydraulic servomechanism 70 is connected to adrain through the D inhibitor valve 58 and the fifth shift valve 68. Asa result, without any axially acting force, the rod 71 of theforward/reverse selection hydraulic servomechanism 70 remains in thesame condition which has existed before the transmission takes thisSecond NEUTRAL mode. No force is generated in the axial direction untilthe spool 68 a of the fifth shift valve 68 is shifted rightward.

Now, each mode for the forward drive range (D range) is described. TheIn-gear mode takes place, for example, when the shift lever ismanipulated from the N position to the D position to engage gears, andthe In-gear mode prepares the transmission to start the engagement ofthe LOW clutch 11. In this mode, the second and third on/off solenoidvalves 82 and 83 are turned on and are open while the first, fourth andfifth on/off solenoid valves 81, 84 and 85 are turned off and areclosed. In response to this actuation condition of the solenoid valves,the shift valves are set as follows: the spool 60 a of the first shiftvalve 60 is shifted rightward, the spool 62 a of the second shift valve62 is shifted leftward, the spool 64 a of the third shift valve 64 isshifted leftward, the spool 66 a of the fourth shift valve 66 is shiftedleftward, and the spool 68 a of the fifth shift valve 68 is shiftedleftward.

In the In-gear mode, the LOW clutch 11 is controlled to engage graduallyby the first linear solenoid valve 86. The hydraulic pressure adjustedby the first linear solenoid valve 86 is supplied to the oil passage 107which is connected with the oil passage 108 through the CPB valve 56.This oil passage 108 is connected with another oil passage 128 through apassage which is created when the spool 68 a of the fifth shift valve 68is shifted leftward. The oil passage 128 is then connected with anotheroil passage 129 through a passage which is created when the spool 64 aof the third shift valve 64 is shifted leftward. Then, the oil passage129 is connected with another oil passage 130 through a passage which iscreated when the spool 62 a of the second shift valve 62 is shiftedleftward. This oil passage 130 is then connected with another oilpassage 131 through a passage which is created when the spool 66 a ofthe fourth shift valve 66 is shifted leftward. This oil passage 131 isthen connected to the oil chamber of the LOW clutch 11 and to the LOWaccumulator 75. In this arrangement, the LOW clutch 11 is engagedgradually in correspondence to the activation of the first linearsolenoid valve 86.

In the In-gear mode, the oil passage 125 connected to the right side oilchamber 73 of the forward/reverse selection hydraulic servomechanism 70is connected with the oil passage 126 through a passage which is createdby the leftward shift of the spool 66 a of the fourth shift valve 66.This oil passage 126 is then connected with the oil passage 135 whichleads to the oil passage 101 e, through a passage created by therightward shift of the spool 58 a of the D inhibitor valve 58. On theother hand, the passage 106 connected to the left side oil chamber 72 ofthe forward/reverse selection hydraulic servomechanism 70 is connectedthrough the D inhibitor valve 58 with the oil passage 104 which isdrained at the fifth shift valve 68 whose spool 68 a is shifted on theleft side thereof. As a result, the line pressure PL being supplied intothe right side oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70 acts on the rod 71 and pushes it leftward. Therefore,in the In-gear mode, the rod 71 of the forward/reverse selectionhydraulic servomechanism 70 is shifted leftward as shown in the figure,so the dog clutch 16 shifts to the D range position and engages with theFOURTH speed driven gear 24 b to connect it rotationally to thecountershaft 3.

When the dog clutch 16 is shifted to the D range position (forward driverange position) in this way, the line pressure is supplied through theoil passage 126 a branching from the oil passage 126 to the hydraulicswitch 93, and the hydraulic switch 93 is turned on. In other words,whether the line pressure, which establishes the D range, is suppliedinto the right side oil chamber 73 of the forward/reverse selectionhydraulic servomechanism 70 or not is detectable by the hydraulic switch93.

Now, a description is given of the LOW mode. In the LOW mode, which isset, for example, to start the vehicle when the D range is selected, thefirst˜third on/off solenoid valves 81˜83 are turned on and are openedwhile the fourth and fifth on/off solenoid valves 84 and 85 are turnedoff and are closed. In this condition, the spool 60 a of the first shiftvalve 60 is shifted to the left side, the spool 62 a of the second shiftvalve 62 is shifted to the left side, the spool 64 a of the third shiftvalve 64 is shifted to the left side, the spool 66 a of the fourth shiftvalve 66 is shifted to the left side, and the spool 68 a of the fifthshift valve 68 is shifted to the left side.

The LOW mode differs from the In-gear mode only in the actuation of thefirst on/off solenoid valve 81. In the LOW mode, the first on/offsolenoid valve 81 is turned on, so the spool 60 a of the first shiftvalve 60 is shifted leftward. Then, the line pressure PL being suppliedfrom the first on/off solenoid valve 81 to the oil passage 122 is ledthrough the oil passage 105 to the left end of the D inhibitor valve 58,so the spool 58 a of the D inhibitor valve 58 is shifted rightward. Inthis condition, the oil passage 135 branching from the oil passage 101e, to which the line pressure PL is being supplied, is connected withthe oil passage 126 through the D inhibitor valve 58, so the linepressure PL is now supplied to the D inhibitor valve 58 through the oilpassage 126.

In the condition where the oil passage 135 is connected with the oilpassage 126, the spool 58 a of the D inhibitor valve 58 is pushedrightward by the line pressure PL being supplied, so the spool 58 aremains on the right side of the valve even after the line pressuresupplied through the oil passage 105 is terminated. This spool 58 aremains on the right side unless the line pressure from the oil passage139 acts on the spool 58 a and pushes leftward, which pressure is onlyavailable when the fourth on/off solenoid valve 84 is turned on to shiftthe spool 68 a of the fifth shift valve 68 rightward. Therefore, oncethe spool 58 a of the D inhibitor valve 58 is shifted rightward, itremains on the right side until the fourth on/off solenoid valve 84 isturned on.

Now, the oil passage 126 is connected with the oil passage 125 through apassage created by the leftward shift of the spool 66 a of the fourthshift valve 66, so the line pressure PL is supplied through the oilpassage 125 to the right side oil chamber 73 of the forward/reverseselection hydraulic servomechanism 70. As a result, the rod 71 in thisvalve is shifted leftward, so the dog clutch 16 is positioned at the Drange position, engaging with the FOURTH speed driven gear 24 b andconnecting it rotationally to the countershaft 3. Here, the linepressure being supplied into the right side oil chamber 73 is detectedby the hydraulic switch 93.

In the condition where the rod 71 is on the left side, the right sideoil chamber 73 is connected to an oil passage 138 through which the linepressure PL is supplied to the second and third linear solenoid valves87and 88. Now, the line pressure PL is adjustable with the second andthird linear solenoid valves 87and 88, and this adjusted pressure can besupplied as control pressures to oil passages 140 and 142, respectively.However, no control pressure is output from these linear solenoid valves87and 88 in the LOW mode.

In the LOW mode, the control pressure being supplied from the firstlinear solenoid valve 86 to the oil passage 107 is led to the LOW clutch11 in the same way as in the In-gear mode. Therefore, the engagement ofthe LOW clutch 11 is controlled in correspondence to the actuation ofthe first linear solenoid valve 86.

Now, a description is given of the 1-2-3 mode. This mode is set to shiftthe speed ratio of the transmission among the first (LOW), second, andTHIRD speed ratios, i.e., to control the transition of speed ratiochange. In this mode, the second and third on/off solenoid valves 82 and83 are turned on and are opened while the first and fourth on/offsolenoid valves 81 and 84 are turned off and are closed. The fifthon/off solenoid valve 85 is turned off when the FIRST speed ratio isestablished, and it is turned on or off in lock-up clutch actuationcontrol (no description is provided on this control because it is notrelevant to the present invention) when the second or THIRD speed ratiois established. In the 1-2-3 mode, the spool 60 a of the first shiftvalve 60 is shifted rightward, the spool 62 a of the second shift valve62 is shifted leftward, the spool 64 a of the third shift valve 64 isshifted leftward, and the spool 66 a of the fourth shift valve 66 isshifted leftward.

As the fourth on/off solenoid valve 84 is turned off, the spool 58 a ofthe D inhibitor valve 58 is maintained to the right side. In thiscondition, the line pressure PL is supplied to the right side oilchamber 73 of the forward/reverse selection hydraulic servomechanism 70,so the dog clutch 16 is maintained at the D range position. Here, theline pressure being supplied into the right side oil chamber 73 isdetected by the hydraulic switch 93. In this condition, the linepressure PL is also supplied through the oil passage 138 to the secondand third linear solenoid valves 87 and 88.

In this mode, the engagement of the LOW clutch 11, the SECOND speedclutch 12 or the THIRD speed clutch 13 is controlled in correspondenceto the actuation of the first, second and third linear solenoid valve86, 87 and 88. The control pressure being supplied from the first linearsolenoid valve 86 to the oil passage 107 is led to the oil passage 108through the CPB valve 56. This oil passage 108 is connected with the oilpassage 128 through the fifth shift valve 68, and this oil passage 128is connected with the oil passage 129 through the third shift valve 64.Then, this oil passage 129 is connected with the oil passage 130 throughthe second shift valve 62, and this oil passage 130 is connected withthe oil passage 131 through the fourth shift valve 66. This oil passage131 is then connected to the LOW clutch 11. In this condition, theengagement of the LOW clutch 11 is controlled by means of the controlpressure supplied from the first linear solenoid valve 86.

The primary pressure of the second linear solenoid valve 87 is thepressure supplied through the oil passage 138, which pressure issupplied only when the forward/reverse selection hydraulicservomechanism 70 is set at the D range position. This primary pressureis adjusted by the second linear solenoid valve 87 to a control pressurewhich is supplied to the oil passage 140. In this present condition,this oil passage 140 is connected through the third shift valve 64 withan oil passage 145, which is connected with an oil passage 146 throughthe first shift valve 60. This oil passage 146 is then connected withanother oil passage 147 through the second shift valve 62, and this oilpassage 147 is then connected with another oil passage 148 through thefirst shift valve 60. Then, this oil passage 148 is connected withanother oil passage 149 through the fourth shift valve 66, and this oilpassage 149 is connected to the SECOND speed clutch 12, a hydraulicswitch 92 and the SECOND accumulator 76. In this arrangement, thecontrol pressure from the second linear solenoid valve 87 is utilizedfor the engagement control of the SECOND speed clutch 12.

In this way, the pressure generated only when the forward/reverseselection hydraulic servomechanism 70 is at the D range position issupplied to the second linear solenoid valve 87 as the primary pressure,and this supply of the pressure is detected by the hydraulic switch 92.In other words, a condition where the forward/reverse selectionhydraulic servomechanism 70 is set at the D range position is detectableby the hydraulic switch 92.

The control pressure generated at the third linear solenoid valve 88 issupplied to the oil passage 142 which is connected with another oilpassage 150 through the first shift valve 60. This oil passage 150 isconnected through the third shift valve 64 with another oil passage 151which is connected to the THIRD speed clutch 13 and the THIRDaccumulator 77. As a result, the control pressure form the third linearsolenoid valve 88 is utilized for the engagement control of the THIRDspeed clutch 13.

Now, a description is given of the SECOND mode, which is set for theengagement of the SECOND speed clutch 12. In this mode, the secondon/off solenoid valve 82 is turned on and is opened while the first,third and fourth on/off solenoid valves 81, 83 and 84 are turned off andare closed. The fifth on/off solenoid valve 85 is turned on or offdepending upon the condition of the lock-up clutch actuation control. Inthis condition, the spool 60 a of the first shift valve 60 is shiftedrightward, the spool 62 a of the second shift valve 62 is shiftedleftward, the spool 64 a of the third shift valve 64 is shiftedrightward, and the spool 66 a of the fourth shift valve 66 is shiftedleftward.

As the fourth on/off solenoid valve 84 is turned off also in this mode,the spool 58 a of the D inhibitor valve 58 is maintained on the rightside. Therefore, the line pressure PL is supplied to the right side oilchamber 73 of the forward/reverse selection hydraulic servomechanism 70,so the dog clutch 16 is retained at the D range position. Here, the linepressure being supplied into the right side oil chamber 73 is detectedby the hydraulic switch 93. In this condition, the line pressure PL issupplied through the oil passage 138 to the second and third linearsolenoid valves 87 and 88.

In this mode, the engagement of the SECOND speed clutch 12 is controlledby the control pressure supplied from the second linear solenoid valve87 to the oil passage 140. This oil passage 140 is connected through thethird shift valve 64 with the oil passage 145, which is connected withthe oil passage 146 through the first shift valve 60. This oil passage146 is then connected with the oil passage 147 through the second shiftvalve 62, and this oil passage 147 is connected with the oil passage 148through the first shift valve 60. Furthermore, this oil passage 148 isconnected with the oil passage 149 through the fourth shift valve 66,and this oil passage 149 is connected to the SECOND speed clutch 12 andto the SECOND accumulator 76. In this arrangement, the engagement of theSECOND speed clutch 12 is controlled by the control pressure suppliedfrom the second linear solenoid valve 87.

As described above, the primary pressure of the second linear solenoidvalve 87, which is supplied from the oil passage 138, is generated onlywhen the forward/reverse selection hydraulic servomechanism 70 is at theD range position. This primary pressure is controlled or adjusted at thesecond linear solenoid valve 87 and then supplied into the oil passage140. In this way, the control pressure which is generated from theprimary pressure at the second linear solenoid valve 87 is supplied tothe SECOND speed clutch 12, and the supply of this control pressure isdetected by the hydraulic switch 92. As a result, a condition where theforward/reverse selection hydraulic servomechanism 70 is set at the Drange position is confirmed through the actuation of the hydraulicswitch 92.

Here, the control of the lock-up clutch performed by the fifth on/offsolenoid valve 85 is described briefly. By the turning on or off of thissolenoid valve 85, the position of the spool 68 a of the fifth shiftvalve 68 is controlled leftward or rightward, respectively. In thecondition where this spool 68 a is shifted leftward, the oil passage 101e is connected with another oil passage 155, and the line pressure PL issupplied to the left end of the lock-up shift valve 51. On the otherhand, in the condition where the spool 68 a is shifted rightward, theoil passage 155 is connected to a drain at the fifth shift valve 68, sono pressure is supplied to the left end of the lock-up shift valve 51.In this way, the turning on and off of the fifth on/off solenoid valve85 is utilized for the control of the actuation of the lock-up shiftvalve 51.

The lock-up shift valve 51 is a valve to turn on or off the lock-upactuation, and the engagement of the lock-up clutch is controlled by thecontrol pressure supplied from the first linear solenoid valve 86. Thecontrol pressure from the first linear solenoid valve 86 is led to theoil passage 107, which is connected through another oil passage 157 tothe lock-up control valve 52. Thus, the actuation of the lock-up controlvalve 52, which is controlled by the control pressure from the firstlinear solenoid valve 86, controls the engagement of the lock-up clutch.This engagement control of the lock-up clutch is carried out in the sameway for the speed change ratios which are equal to or higher than theSECOND speed ratio.

Now, a description is given of the THIRD mode, which is set for theengagement of the THIRD speed clutch 13. In this mode, the first˜fourthon/off solenoid valves 81˜84 are turned off and are closed. The fifthon/off solenoid valve 85 is turned on or off depending upon thecondition of the lock-up clutch actuation control as in the abovedescribed mode. In this condition, the spool 60 a of the first shiftvalve 60 is shifted rightward, the spool 62 a of the second shift valve62 is shifted rightward, the spool 64 a of the third shift valve 64 isshifted rightward, and the spool 66 a of the fourth shift valve 66 isshifted leftward.

As the fourth on/off solenoid valve 84 is turned off also in this mode,the spool 58 a of the D inhibitor valve 58 is maintained on the rightside. Therefore, the line pressure PL is supplied to the right side oilchamber 73 of the forward/reverse selection hydraulic servomechanism 70,so the dog clutch 16 is retained at the D range position. Here, the linepressure being supplied into the right side oil chamber 73 is detectedby the hydraulic switch 93. In this condition, the line pressure PL issupplied through the oil passage 138 to the second and third linearsolenoid valves 87 and 88.

In this mode, the engagement of the THIRD speed clutch 13 is controlledby the control pressure supplied from the third linear solenoid valve 88to the oil passage 142, which is connected with another oil passage 160through the first shift valve 60. This oil passage 160 is then connectedwith another oil passage 161 through the second shift valve 62, and thisoil passage 161 is then connected through the third shift valve 64 withthe oil passage 151, which is connected to the THIRD speed clutch 13 andto the THIRD accumulator 77. In this arrangement, the engagement of theTHIRD speed clutch 13 is controlled by the control pressure from thethird linear solenoid valve 88.

Here, the primary pressure of the third linear solenoid valve 88, whichis supplied from the oil passage 138, is generated only when theforward/reverse selection hydraulic servomechanism 70 is at the D rangeposition. This primary pressure is controlled or adjusted at the thirdlinear solenoid valve 88 and then supplied into the oil passage 142. Inthis way, the control pressure generated from the primary pressure ofthe third linear solenoid valve 88, which primary pressure is availableonly when the forward/reverse selection hydraulic servomechanism 70 isat the D range position, is supplied to the THIRD speed clutch 13. Thissupply of this control pressure is detected by another hydraulic switch91, so a condition where the forward/reverse selection hydraulicservomechanism 70 is set at the D range position is confirmed throughthe actuation of the hydraulic switch 91.

Now, a description is given of the 2-3-4 mode. This mode is set to shiftthe speed ratio of the transmission among the second, third and FOURTHspeed ratios, i.e., to control the transition of speed ratio change. Inthis mode, the third on/off solenoid valve 83 is turned on and is openedwhile the first, second and fourth on/off solenoid valves 81, 82 and 84are turned off and are closed. The fifth on/off solenoid valve 85 isused in the lock-up clutch actuation control as briefly described above.In the 2-3-4 mode, the spool 60 a of the first shift valve 60 is shiftedrightward, the spool 62 a of the second shift valve 62 is shiftedrightward, the spool 64 a of the third shift valve 64 is shiftedleftward, and the spool 66 a of the fourth shift valve 66 is shiftedleftward.

Also in this mode, as the fourth on/off solenoid valve 84 is turned off,the spool 58 a of the D inhibitor valve 58 is maintained on the rightside thereof. In this condition, the line pressure PL is supplied to theright side oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70, so the dog clutch 16 is retained at the D rangeposition. Here, the line pressure being supplied into the right side oilchamber 73 is detected by the hydraulic switch 93. In this condition,the line pressure PL is supplied through the oil passage 138 to thesecond and third linear solenoid valves 87 and 88.

In this mode, the engagement of the SECOND speed clutch 12, the THIRDspeed clutch 13 and the FOURTH speed clutch 14 is controlled incorrespondence to the actuation of the first, second and third linearsolenoid valves 86, 87 and 88 to shift the transmission smoothly amongthese speed change ratios.

The control pressure supplied from the first linear solenoid valve 86 tothe oil passage 107 is led to the oil passage 109 through the CPB valve56. This oil passage 108 is connected through the fifth shift valve 68with the oil passage 128, which is connected with the oil passage 129through the third shift valve 64. Then, this oil passage 129 isconnected with the oil passage 147 through the second shift valve 62,and this oil passage 147 is connected with the oil passage 148 throughthe first shift valve 60. This oil passage 148 is then connected throughthe fourth shift valve 66 with the oil passage 149, which is connectedto the SECOND speed clutch 12. In this arrangement, the engagement ofthe SECOND speed clutch 12 is controlled by the control pressuresupplied from the first linear solenoid valve 86.

The control pressure from the second linear solenoid valve 87 is led tothe oil passage 140, which is connected with the oil passage 113 throughthe third shift valve 64. This oil passage 113 is connected with the oilpassage 114 through the second shift valve 62, and this oil passage 114is connected to the FOURTH speed clutch 14 and to the FOURTH accumulator78. In this arrangement, the engagement of the FOURTH speed clutch 14 iscontrol by the control pressure supplied from the second linear solenoidvalve 87.

The control pressure from the third linear solenoid valve 88 is suppliedto the oil passage 142, which is connected with the oil passage 150through the first shift valve 60. This oil passage 150 is then connectedwith the oil passage 151 through the third shift valve 64, and then thisoil passage 151 is connected to the THIRD speed clutch 13 and to theTHIRD accumulator 77. In this arrangement, the engagement control of theTHIRD speed clutch 13 is controlled by the control pressure form thethird linear solenoid valve 88.

Now, a description is given of the FOURTH mode, which is set for theengagement of the FOURTH speed clutch 14. In this mode, the first andthird on/off solenoid valves 81 and 83 are turned on and are openedwhile the second and fourth on/off solenoid valves 82 and 84 are turnedoff and are closed. The fifth on/off solenoid valve 85 is turned on oroff depending upon the condition of the lock-up clutch actuation controlas described above. In this condition, the spool 60 a of the first shiftvalve 60 is shifted leftward, the spool 62 a of the second shift valve62 is shifted rightward, the spool 64 a of the third shift valve 64 isshifted leftward, and the spool 66 a of the fourth shift valve 66 isshifted leftward.

Also, in this mode, as the fourth on/off solenoid valve 84 is turnedoff, the spool 58 a of the D inhibitor valve 58 is retained on the rightside thereof. As a result, the line pressure PL is supplied to the rightside oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70, so the dog clutch 16 is retained at the D rangeposition. Here, the line pressure being supplied into the right side oilchamber 73 is detected by the hydraulic switch 93. The line pressure PLis also supplied through the oil passage 138 to the second and thirdlinear solenoid valves 87 and 88.

In this mode, the engagement of the FOURTH speed clutch 14 is controlledby the control pressure supplied from the second linear solenoid valve87 to the oil passage 140, which is connected with the oil passage 113through the third shift valve 64. This oil passage 113 is connected withthe oil passage 114 through the second shift valve 62, and this oilpassage 114 is then connected to the FOURTH speed clutch 14 and to theFOURTH accumulator 78. In this arrangement, the engagement of the FOURTHspeed clutch 14 is controlled by the control pressure from the secondlinear solenoid valve 87.

Now, a description is made of the 4-5 mode, which is set to shift thespeed ratio of the transmission between the fourth and FIFTH speedratios, i.e., to control the transition of speed ratio change. In thismode, the first on/off solenoid valve 81 is turned on and is openedwhile the second˜fourth on/off solenoid valves 82, 83 and 84 are turnedoff and are closed. The fifth on/off solenoid valve 85 is used in thelock-up clutch actuation control. In the 4-5 mode, the spool 60 a of thefirst shift valve 60 is shifted leftward, the spool 62 a of the secondshift valve 62 is shifted rightward, the spool 64 a of the third shiftvalve 64 is shifted rightward, and the spool 66 a of the fourth shiftvalve 66 is shifted leftward.

Also in this mode, as the fourth on/off solenoid valve 84 is turned off,the spool 58 a of the D inhibitor valve 58 is retained on the right sidethereof. In this condition, the line pressure PL is supplied to theright side oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70, so the dog clutch 16 is retained at the D rangeposition. Here, the line pressure being supplied into the right side oilchamber 73 is detected by the hydraulic switch 93. Furthermore, the linepressure PL is supplied through the oil passage 138 to the second andthird linear solenoid valves 87 and 88.

In this mode, the engagement of the FOURTH speed clutch 14 and of theFIFTH speed clutch 15 is controlled in correspondence to the actuationof the second and third linear solenoid valves 87 and 88, respectively,to change the speed ratio of the transmission smoothly.

The control pressure from the second linear solenoid valve 87 is led tothe oil passage 140. This oil passage 140 is connected with the oilpassage 113 through the third shift valve 64, and this oil passage 113is connected through the second shift valve 62 with the oil passage 114,which is connected to the FOURTH speed clutch 14 and to the FOURTHaccumulator 78. In this arrangement, the engagement of the FOURTH speedclutch 14 is controlled by the control pressure from the second linearsolenoid valve 87.

On the other hand, the control pressure from the third linear solenoidvalve 88 is led to the oil passage 142, which is connected with anotheroil passage 170 through first shift valve 60. This oil passage 170 isthen connected through the third shift valve 64 with another oil passage171, which is connected to the FIFTH speed clutch 15 and to the FIFTHaccumulator 79. In this arrangement, the engagement of the FIFTH speedclutch 15 is controlled by the control pressure from the third linearsolenoid valve 88.

Now, a description is made of the FIFTH mode, which is set for theengagement of the FIFTH speed clutch 15. In this mode, the first andsecond on/off solenoid valves 81 and 82 are turned on and are openedwhile the third and fourth on/off solenoid valves 83 and 84 are turnedoff and are closed. The fifth on/off solenoid valve 85 is turned on oroff depending upon the condition of the lock-up clutch actuation controlas described above. In this condition, the spool 60 a of the first shiftvalve 60 is shifted leftward, the spool 62 a of the second shift valve62 is shifted leftward, the spool 64 a of the third shift valve 64 isshifted rightward, and the spool 66 a of the fourth shift valve 66 isshifted leftward.

Also, in this mode, as the fourth on/off solenoid valve 84 is turnedoff, the spool 58 a of the D inhibitor valve 58 is retained on the rightside thereof. As a result, the line pressure PL is supplied to the rightside oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70, so the dog clutch 16 is retained at the D rangeposition. Here, the line pressure being supplied into the right side oilchamber 73 is detected by the hydraulic switch 93. The line pressure PLis also supplied through the oil passage 138 to the second and thirdlinear solenoid valves 87 and 88.

In this mode, the engagement of the FIFTH speed clutch 15 is controlledby the control pressure supplied from the third linear solenoid valve 88to the oil passage 142, which is connected with the oil passage 170through the first shift valve 60. This oil passage 170 is connectedthrough the third shift valve 64 with the oil passage 171, which isconnected to the FIFTH speed clutch 15 and to the FIFTH accumulator 79.In this arrangement, the engagement of the FIFTH speed clutch 15 iscontrolled by the control pressure from the third linear solenoid valve88.

As described above, each mode is established by controlling the turningon and off of the first˜fifth on/off solenoid valves 81˜85 as listed inTable 1. The alphabets on the left side of Table 1, “R”, “N” and “D”,represent the reverse drive range, the neutral range and the forwarddrive range, respectively, which are switched one after another incorrespondence to the manipulation of the shift lever at the driver'sseat. For example, in a case where the shift lever is manipulated toshift the range setting of the transmission from the R range through theN range to the D range, at first, the Second NEUTRAL mode is set as theN range. In this N range condition, the rod 71 of the forward/reverseselection hydraulic servomechanism 70 is retained at the reverse driveposition without any force acting in the axial direction. Thereafter,when the shift lever is manipulated to the D range, the control systemproceeds to set the transmission into the In-gear mode and then into theLOW mode.

On the other hand, in a case where the shift lever is manipulated toswitch the range of the transmission from the D range to the N range andthen from the N range to the R range, if the speed of the vehicle at thetime of the switching to the N range is less than a critical speed orpredetermined speed (e.g., 10 km/h), then the control system sets theSecond NEUTRAL mode. In this N range condition, the rod 71 of theforward/reverse selection hydraulic servomechanism 70 is retained at theforward drive position without any force acting in the axial direction.Thereafter, when the shift lever is manipulated to the R range, thecontrol system proceeds to set the transmission into the REVERSE mode.

On the other hand, if the speed of the vehicle at the time of theswitching to the N range is equal to or more than the critical speed,then the control system sets the First NEUTRAL mode. As described above,in this mode, the rod 71 of the forward/reverse selection hydraulicservomechanism 70 is retained at the forward drive position with a forceacting in the axial direction toward the forward drive position, whichforce is generated by the line pressure supplied into the right side oilchamber 73. In this condition, even if a solenoid valve malfunctions andengages a corresponding clutch, the reverse speed ratio cannot beestablished. When the speed of the vehicle decreases below the criticalspeed, the First NEUTRAL mode is switched to the Second NEUTRAL mode.However, if the shift lever is manipulated to set the R range while thevehicle is driving still at a speed equal to or more than the criticalspeed, then the control system retains the First NEUTRAL mode and doesnot proceed to set the REVERSE mode, i.e., the control system providesthe above mentioned inhibitor function. Thereafter, when the vehiclespeed decreases below the critical speed, the control system proceeds toget the transmission into the REVERSE mode.

Finally, a description is made of the F/S (Fail Safe) SECOND mode. Thismode is set to secure a certain driving performance by fixing thetransmission to the SECOND speed ratio when the transmission experiencesa breakdown. In this mode, the first˜fourth on/off solenoid valves 81˜84are turned on and are opened while the fifth on/off solenoid valve 85 isturned off and is closed. In this condition, the spool 60 a of the firstshift valve 60 is shifted leftward, the spool 62 a of the second shiftvalve 62 is shifted leftward, the spool 64 a of the third shift valve 64is shifted leftward, the spool 66 a of the fourth shift valve 66 isshifted rightward, and the spool 68 a of the fifth shift valve 68 isshifted leftward.

In this mode, the engagement of the SECOND speed clutch 12 is controlledby the control pressure supplied from the first linear solenoid valve 86to the oil passage 107, which is connected with the oil passage 108through the CPB valve 56. This oil passage 108 is connected with the oilpassage 128 through the fifth shift valve 68, and this oil passage 128is connected with the oil passage 129 through the third shift valve 64.This oil passage 129 is then connected with the oil passage 130 throughthe second shift valve 62, and this oil passage 130 is connected throughthe fourth shift valve 66 with the oil passage 149, which is connectedto the SECOND speed clutch 12 and to the SECOND accumulator 76. In thisarrangement, the engagement of the SECOND speed clutch 12 is controlledby the control pressure supplied from the first linear solenoid valve86.

It can be understood from the above description that the engagement ofthe second˜FIFTH speed clutches 12˜15 are controlled for setting theSECOND mode and higher modes (excluding the F/S mode), respectively, bythe control pressures which are supplied from the second and thirdlinear solenoid valves 87 and 88. The primary pressures supplied to thesecond and third linear solenoid valves 87 and 88 are led through theforward/reverse selection hydraulic servomechanism 70. For example, ifthe forward/reverse selection hydraulic servomechanism 70, the Dinhibitor valve 58, which controls the supply of the line pressure intothe right side oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70, or the fourth shift valve 66 experiences anoperational failure, then these clutches cannot be controlledsystematically. However, the engagement of the SECOND speed clutch 12 inthe F/S (Fail Safe) SECOND mode is controlled by the first linearsolenoid valve 86, which utilizes the line pressure PL being supplieddirectly from the oil passage 100 b bypassing the forward/reverseselection hydraulic servomechanism 70. Therefore, the SECOND speed ratiocan be established regardless of any operational failure of theforward/reverse selection hydraulic servomechanism 70.

This control system is constructed to detect failures, and when thesystem detects a failure, it automatically switches the operational modeof the transmission into the F/S SECOND mode to secure a certain drivingperformance. For detecting failures, a plurality of hydraulic switches91, 92 and 93 are provided and arranged as shown in the figures. Onehydraulic switch 91 detects the pressure of the THIRD speed clutch,another hydraulic switch 92 detects the pressure of the SECOND speedclutch, and the other hydraulic switch 93 detects the pressure of theright side oil chamber 73 of the forward/reverse selection hydraulicservomechanism 70. In addition, the control system detects the actuationsignals of the first˜fifth on/off solenoid valves 81˜85 and continuallymonitors what mode the transmission is in.

In this arrangement, the hydraulic switch 93 detects the pressure whichis supplied into the right side oil chamber 73 of the forward/reverseselection hydraulic servomechanism 70 while the hydraulic switches 91and 92 detect the pressures that control the third and second speedclutches, respectively, which pressures are generated from the pressuresupplied into the right side oil chamber 73 of the forward/reverseselection hydraulic servomechanism 70. Therefore, by these hydraulicswitches 91, 92 and 93, the system can detect at which side, i.e., thereverse drive range or the forward drive range, the forward/reverseselection hydraulic servomechanism 70 is set. In this way, the systemmonitors the condition of the transmission from the result of thedetection by the hydraulic switches and also from the above mentionedactuation signals of the first˜fifth on/off solenoid valves 81˜85. As aresult, the system is capable of determining a failure if it happens.For example, if the line pressure exists in the left side oil chamber72, indicating the dog clutch 16 being set at the R range position,while the actuation signals of the first˜fifth on/off solenoid valves81˜85 are detected to correspond to one of the modes set for the Drange, the system judges this condition as a failure. In the same way,if the line pressure exists in the right side oil chamber 73, indicatingthe dog clutch 16 being set at the D range position, while the actuationsignals of the first˜fifth on/off solenoid valves 81˜85 are detected tocorrespond to a mode for the R range, the system also judges thiscondition as a failure.

The control system according to the present invention determines fromthe position of the forward/reverse selection hydraulic servomechanism70 and from the pattern of the actuation signals whether they are inharmony and the transmission is operating normally or not. Therefore, ifany abnormality or breakdown occurs, the system can detect it andcontrol the transmission accordingly, for example, by setting thetransmission into the Fail Safe SECOND mode. Such control is describedin the following in reference to the flowcharts shown in FIG. 11 andthereafter.

The control system performs a fault detection by comparing the result ofthe detection by the hydraulic switches 92 and 93 with the actuationsignals of the first˜fifth on/off solenoid valves 81˜85. It is clearfrom the above description that the determination of faults is carriedout when the transmission is shifting up from the first speed ratio tothe second speed ratio because the hydraulic switch 92 detects thepressure which actuates the SECOND speed clutch 12. FIG. 11 shows thesteps of the fault determination. At first, a determination is madewhether the transmission is shifting up from the first speed ratio tothe second speed ratio or not at Step S1. If it is upshifting, then thecontrol routine proceeds to Step S2, where it waits for a predeterminedtime T1 until the upshift operation completes, and then the controlroutine proceeds to Step S3. On the other hand, if the transmission isnot upshifting, or if the control is waiting for the elapse of thepredetermined time T1, then this turn of the control routine endswithout performing the fault determination.

At Step S3, a determination is made whether the hydraulic switch 92 isOFF or not, i.e., whether the pressure to actuate the SECOND speedclutch 12 is being supplied or not. If the hydraulic switch 92 is ON,then the control routine proceeds to Step S4, where anotherdetermination is made whether the hydraulic switch 93 is OFF or not. Ifthe hydraulic switch 93 is judged being ON at Step S4, meaning that thetransmission is operating normally, then this turn of the control flowsto the end of the routine. On the other hand, if the hydraulic switch 93is judged being OFF at Step S4, then a warning lamp is lighted to notifythe breakdown of the hydraulic switch 93. In this case, the fault isonly of the hydraulic switch 93, so the other parts are judged all rightwithout any fault at a fault confirmation step at Step S12, which isdescribed later in this section. As a result, the transmission iscontrolled in an ordinary driving mode.

On the other hand, if the hydraulic switch 92 is judged being OFF atStep S3, then the control routine proceeds to Step S6, where adetermination is; made whether the hydraulic switch 93 is OFF or not.Here, if the hydraulic switch 93 is judged being OFF, then it isconsidered that the D inhibitor valve 58 is malfunctioning. In thiscase, the breakdown of the D inhibitor valve 58 should be confirmed(additionally, the warning lamp may be lighted indicating theabnormality). However, if the hydraulic switch 93 is judged being ON atStep S6, then the forward/reverse selection hydraulic servomechanism 70(or the fourth shift valve 66) may be malfunctioning. This abnormalityshould be confirmed (additionally, the warning lamp may be lightedindicating the abnormality). The above mentioned results of the faultdetermination are summarized in the following table, Table 2.

TABLE 2 D range 1-2 upshift Hydraulic Switch 93 Fault check ON OFFHydraulic Switch ON Normal Switch 93 92 failure OFF Servomechanism Dinhibitor valve failure failure

Although the fault determination is described only for the D inhibitorvalve 58 and the forward/reverse selection hydraulic servomechanism 70here, the fault determination is executed for various components, andthe results are stored in memory in a format as shown in FIG. 13. If nofault is found for an item, then value “0”, or if any fault is found forthat item, then value “1” is placed in a corresponding bit address ofone of three memory sections, FAIL01, FAIL02 and FAIL03, as shown inFIG. 13. Thus, the existence of faults is immediately recognizable.

Now, a description is given of the speed change control of the automatictransmission according to the present invention in reference to theflowchart of FIG. 12. This speed change control is executed on the basisof the above described fault determination. At first, the controldetermines the drive range which is selected with the shift lever by thedriver at the driver seat. If the transmission is judged as being set inthe D range at Step S11, then the control routine proceeds to Step S12,where a determination is made whether there is any fault or not. Ifthere is a fault which is found in the above described faultdetermination, i.e., at Step S7 or Step S8 in the flowchart shown inFIG. 11, then the existence of a fault is recognized at Step S12. Inthis case, the control routine proceeds to Step S14, where the systemexecutes a D range relief drive control to operate the transmission inthe F/S (Fail Safe) SECOND mode and thereby allows the vehicle to driveat the second speed ratio. On the other hand, if the transmission isjudged as having no fault at Step S12, then the control routine proceedsto Step S13, where the transmission is controlled to operate in the Drange in an ordinary way.

However, if the transmission is judged as not being in the D range atStep S11, the control routine proceeds to Step S15, where anotherdetermination is made whether the transmission is set in the R range ornot. If it is judged not in the R range, then the control routineproceeds to Step S16 because the transmission is in the N range. At StepS16, a determination is made whether the current speed V of the vehicleis equal to or higher than 10 km/h or not. If the vehicle speed V islower than 10 km/h, then the system sets the normal neutral mode, i.e.,the Second NEUTRAL mode, in which the driver can manipulate the shiftlever to select the D range or the R range, at Step S17. However, if thecurrent vehicle speed V is higher than 10 km/h, then the system preventsthe transmission from shifting into the Reverse mode, i.e., sets theFirst NEUTRAL mode, which functions as a reverse inhibitor, at Step S18.

On the other hand, if the transmission is judged as being in the R rangeat Step S15, then the control routine proceeds to Step S19, where adetermination is made whether the current speed V of the vehicle isequal to or higher than 10 km/h or not. If the vehicle speed V is lowerthan 10 km/h, then the control routine proceeds to Step S21, where thesystem controls to maintain the R range. However, if the current vehiclespeed V is higher than 10 km/h, then the control routine proceeds toStep S20, where a determination is made whether the transmission wascontrolled to shift into the REVERSE mode in the last flow of theroutine or not. If the transmission was not controlled into the REVERSEmode in the last flow, the control routine proceeds to Step S18 becausethe control of Step S18 is considered as being still going on.Therefore, the system maintains the First NEUTRAL mode, which functionsas a reverse inhibitor. On the other hand, if the transmission was setinto the REVERSE mode in the last flow, then it is maintained in theREVERSE mode at Step S21.

Now, the D range relief drive control, which is executed at Step S14 ofthe above described control, is explained in reference to FIG. 14. Thiscontrol is executed when there is a fault, i.e., FAIL01≠0, FAIL02≠0, orFAIL03≠0. At Step S31, a determination is made whether or not thevehicle speed V is equal to or lower than a predetermined speed whichcauses an over rotation at the second speed ratio. If the vehicle speedV is higher than this predetermined speed, then the control routineproceeds to Step S55, where the first˜fifth on/off solenoid valves 81˜85are set into condition “OOOOO”. Here, the “O” marks represent the turnedON states of the solenoid valves, each symbol corresponding, in theorder from the left to the right, to the first˜fifth on/off solenoidvalves 81˜85, respectively. Additionally, “X” marks are used torepresent the turned OFF states of solenoid valves. In other words, allthe solenoid valves 81˜85 are turned on at Step S55 to bring thetransmission into the First NEUTRAL mode (refer to Table 1).

On the other hand, if the vehicle speed V is equal to or lower than thepredetermined speed, then the control routine proceeds to Step S32,where a determination is made whether the value of the first memorysection is zero, FAIL01=0, or not. If the value stored at any bitaddress of the first memory section FAIL01 is one, then the value storedin the first memory section is not zero, FAIL01≠0, so the controlroutine proceeds to Step S33, where a determination is made whether anin-gear control is complete or not. If the result of the determinationis that an in-gear control is complete, then the control routineproceeds to Step S34, where the solenoid valves are set into condition“OOOOX”, i.e., the F/S (Fail Safe) SECOND mode (refer to Table 1), andthe transmission is controlled to operate at the second speed ratio bythe actuation of the first linear solenoid valve 86. If the result ofthe determination at Step S33 is that an in-gear control is stillhalfway, then the control routine proceeds to Step S35, where thesolenoid valves are set into condition “OOOOX”, and the in-gear controlis executed by the actuation of the first linear solenoid valve 86.

If the value stored in the first memory section is judged being zero,FAIL01=0, at Step S32, then the control routine proceeds to Step S41,where a determination is made whether the value stored in the secondmemory section is zero, FAIL02=0, or not. If the value stored at any bitaddress of the second memory section FAIL02 is one, then the valuestored in the second memory section is not zero, FAIL02≠0, so thecontrol routine proceeds to Step S42, where a determination is madewhether an in-gear control is complete or not. If the result of thedetermination is that an in-gear control is complete, then the controlroutine proceeds to Step S43, where the solenoid valves are set intocondition “XXOXX”, i.e., the 2-3-4 mode (refer to Table 1), and thetransmission is controlled to operate at the second speed ratio by theactuation of the first linear solenoid valve 86. On the other hand, ifthe result of the determination is that an in-gear control is stillhalfway, then the control routine proceeds to Step S44, where thesolenoid valves are set into condition “XXOXX”, and the in-gear controlis executed by the actuation of the first linear solenoid valve 86.

However, if the value stored in the second memory section is judgedbeing zero, FAIL02=0, at Step S41, indicating that the value stored inthe third memory section is not zero, FAIL03≠0, then the control routineproceeds to Step S51, where a determination is made whether an in-gearcontrol is complete or not. If the result of the determination is thatan in-gear control is complete, then the control routine proceeds toStep S52, where the solenoid valves are set into condition “XOXXX”,i.e., the SECOND mode (refer to Table 1), and the transmission iscontrolled to operate at the second speed ratio by the actuation of thesecond linear solenoid valve 87. On the other hand, if the result of thedetermination is that an in-gear control is still halfway, then thesolenoid valves are set into condition “XOOXX”, i.e., the In-gear mode(refer to Table 1). Then, after the forward/reverse selection hydraulicservomechanism 70 is actuated, the solenoid valves are set intocondition “XOXXX”, and the in-gear control is executed by the actuationof the second linear solenoid valve 87.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent ApplicationNo.11-208030 filed on Jul. 22, 1999, which is incorporated herein byreference.

What is claimed is:
 1. A control system for an automatic transmissionwhich comprises a power transmission mechanism incorporating a pluralityof power transmission paths and a plurality of hydraulically operatedfrictionally engaging means for selecting said power transmission pathsindividually, a plurality of shift control valves for controlling supplyof a hydraulic pressure to said hydraulically operated frictionallyengaging means, and a forward/reverse hydraulic servomechanism forselecting either said power transmission paths which belong to a forwarddrive range or said power transmission paths which belong to a reversedrive range; wherein: said control system comprises a plurality ofsolenoid valves for supplying and draining a line pressure which is usedto actuate said shift control valves and said forward/reverse hydraulicservomechanism for selecting said power transmission paths in a speedchange control; said hydraulic pressure supplied to one of saidhydraulically operated frictionally engaging means which establishes acertain speed ratio for said forward drive range is generated from thehydraulic pressure which is supplied into a forward drive servo-oilchamber of said forward/reverse hydraulic servomechanism; said controlsystem further comprises first pressure detecting means and secondpressure detecting means, said first pressure detecting means detectingthe hydraulic pressure which is supplied to said forward drive servo-oilchamber for selecting said power transmission paths belonging to saidforward drive range, and said second pressure detecting means detectingsaid hydraulic pressure which is supplied to said one of saidhydraulically operated frictionally engaging means; and said controlsystem determines whether said forward drive range is set correctly ornot, based on a result of pressure detection by said first and secondpressure detecting means.
 2. The control system as set forth in claim 1,wherein: a D inhibitor valve and a predetermined shift control valve,each of which is retainable at a respective forward drive position forsaid forward drive range, are provided on a servo-pressure supply linewhich connects a source of said line pressure to said forward driveservo-oil chamber, with said D inhibitor valve being positioned closerto said line pressure source, such that said servo-pressure supply lineis in fluid communication when said D inhibitor valve and saidpredetermined shift control valve are positioned at said respectiveforward drive positions; and said first pressure detecting means detectsthe hydraulic pressure of said servo-pressure supply line at a locationbetween said D inhibitor valve and said predetermined shift controlvalve.
 3. The control system as set forth in claim 1, wherein: saidhydraulic pressure supplied to said one of said hydraulically operatedfrictionally engaging means, which establishes a certain speed ratio forsaid forward drive range, is generated by a linear solenoid valve whichadjusts said hydraulic pressure supplied to said forward drive servo-oilchamber.
 4. The control system as set forth in claim 3, wherein: saidsecond pressure detecting means detects the hydraulic pressure which issupplied from said forward drive servo-oil chamber to said linearsolenoid valve.
 5. The control system as set forth in claim 1 or claim2, wherein: when said forward drive range is judged not being setcorrectly, based on said result of pressure detection by said first andsecond pressure detecting means, said control system controls saidsolenoid valves in accordance to modes which are predetermined forpossible faults, and thereby sets said transmission into a relief drivemode.